SN interstellar reddening

[nytecam]

Hi - we had a very interesting, if deep, talk last night at Ewell AS by Dutch astronomer Dr Paul Kuin at UCL Mullards Space Science Labs Dorking on novae and supernova and interstellar reddening [iRed] of the later sighting SN2014J [M82] and SN2011fe? [M101] both in Uma. He claimed IRed was marked with M82 but virtually absent with M101 SN and pros are puzzled as to why!

As the only SN discoverer present the Chairman gave me the option of the first question! I thought the reddening in SN2014J was local to the galaxy and that as M101 was a face-on galaxy, reddening would be minimal. If the reddening was interstellar then Doppler Shift of its lines would locate it between us and the SN but didn't get a satisfactory answer. What's the general view on SNe reddening? TIA

[robin_astro]

Hi Maurice,

There is not a direct correlation between the amount of redenning of a supernova and that of the parent galaxy as a whole. it all depends where in the parent galaxy the sn is located.

Light from Supernovae suffers redenning on its way to use from three distinct sources (in reverse order) :- 

1. redenning from the passage of the light through our own galaxy

2. intergalactic redenning 

3. redenning as the light passes out  through the parent galaxy

In the case of sn2014J most of the redenning comes from 3 ie from within M82 itself (If the sn happened to be on the edge of  of M82 facing us the redenning would have been be much less)

M82 is seen side on so for example a sn on the opposite side to us would probably not even be visible from here due to the high extinction passing through the whole of the galaxy. (There is even a theory that we may have missed sn on the opposite side of our own galaxy due to high extinction from dust in the direction of the galactic centre) As M101 is seen face on though,  the redenning from 3 would be less in general and fairly similar independent of the location of the sn

HTH

Robin

[robin_astro]

You are right though that it should be possible to determine the origin of the redenning from the imprint of absorption lines. The sn2014J spectrum showed very strong sharp Na D interstellar absorption lines as seen in many of the amateur spectra here.

http://www.astrosurf.com/aras/Aras_DataBase/Supernovae/SN2014J.htm

Confirming that they had the same redshift of the galaxy (plus or minus the rotational  velocity of M82)  should be pretty straightforward given a modest resolution spectrum

Robin

[robin_astro]

A Quick look at amateur Olivier Garde's impressive high resolution (R~11000) spectrum with an echelle spectrograph  here

http://www.astrosurf.com/aras/Aras_DataBase/Supernovae/SN2014J_M82.htm

(spectrum number 40)

shows the redshift of the sn2014J Na D lines to be ~2.6A or around 150km/s . This compares with the published figure for M82 as a whole of 219km/s so consistent with the IS redenning being from M82. The problem is though that the M82 redshift is so small that there may be parts of our own galaxy moving at this sort of speed relative to use so it is not conclusive (Interestingly the Na D lines in Olivier Garde's spectrum are quite wide so the absorption is from material moving at a range of velocities.   

Robin

[robin_astro]

Here are the Na D lines from Olivier Garde's spectrum. (The green lines show  the rest wavelengths of the D lines) 

Robin

[nytecam]

Thanks Robin - I'd hoped you'd give some valid info and you have - many thanks and I'll go with that ;-) Dr Kuin has a strong accent and maybe I didn't hear him too well.

[robin_astro]

Just to put this one completely to bed I checked the rotation radial velocity curve for M82  eg

http://arxiv.org/abs/1202.0824

SN2014J is just to the west of the galactic centre,  where  the rv is negative (increasing from zero to ~ -130km/s in the outer regions) so this should nicely explain the remaining difference between the RV of sn2014J measured from the spectrum (~150km/s) and the mean RV published for the galaxy (219km/s)

Cheers

Robin

[Jack Martin]

Maurice and Robin,

Spectroscopy is always fascinating and this makes very interesting reading.

Regards,

Jack